I'm not sure how criticals work.  I assume it's a straight percent and is unavoidable when it 'procs'.  I also assume that crit damage = damage*(1+x).  Right now I have seen some insane criticals and unfortunately they happen where I either can't backtrack to see what the defense of the target was(not even sure armor is accounted for).  I think a 20 atk vs my 27 armor did 272 dmg, and recently I myself hit a target with my 27 attack for over 700 damage.  The scaling seems odd, ignoring the 3 digits. 

I have a single archer that I use in spare who does around 4 dmg sometimes 7, she has been on the assassin path and only has the innate path bonus to crit%, but she will hit for 200+ on a crit (plain longbow).  My pet dog will typically hit for 1 if that, and while attacking a prone defender crit'ed for 4. 

I don't have better comparisons since it is something hard to duplicate, and when it happens it is an instant death so I can't check the stats on the dead target.